Patent GB-1474597-A (GB)
Title: Pay television system
IPC: H04N7/171
Filed: 1974-05-15
Abstract: 1474597 Television TELEGLOBE PAY TV SYSTEM Inc 15 May 1974 21502/74 Heading H4F A pay television system involves: adding reference pulses, of opposite polarity to the line sync. pulses, on the front porch of each horizontal blanking interval of a standard colour television signal; inverting the line video portions only of randomly selected video frames; adding reset bursts to the first two line periods of the vertical blanking interval; and adding decoding bursts to the last eight line periods of each vertical blanking interval. The standard signal is shown by A, Fig. 1, as having a black negative (B -) level. The line video portion being schematically shown as white, grey and black levels and colour bursts present on the line sync. back porches. The reference pulses of peak level are added where shown in B and randomly selected frames have line video invertion rendering black positive (B+) levels as shown in C. Fig. 2 shows a typical vertical blanking interval wherein the first two line periods contain reset bursts which resets a gate prior to determining the polarity of the line video portions of the subsequent frame. This polarity is determined according to information in the eight lines of frequency coded decoding bursts terminating the vertical blanking interval as seen in Fig. 2. The reference pulses A are present throughout this interval. The audio component is modulated on a 31À5 kHz suppressed carrier while the normal audio carrier is used to transmit a barker audio signal. Fig. 3 shows a coding circuit in which the standard signal is applied to inverting and non-inverting amplifiers 11, 12 gated by a circuit 15 in synchronism with the standard signal and in accordance with the output of a random switching signal generator 14. Amplifiers 11 and 12 add the reference pulses to each line by having their outputs suppressed simultaneously in the relevant intervals, yielding an extremely reliable reference level. Gating generator 15 also controls reset burst gate 16, B - and B + burst gates 17, 18 which pass coded frequency bursts from generators 19 via selector 20, in synchronism with the vertical blanking intervals, to a summing junction for output of the encoded form. Fig. 11 shows a decoding system. The encoded signal is received by a standard unit 700. The reference pulses provide reference levels throughout the decoder. Reference pulses and decoding/reset bursts are separated at 701. The reference pulses are used: at 705 to cancel the reference pulses in the final decoded signal; to activate an audio carrier generator 702 for reinsertion of the audio component; and to generate horizontal blanking 704. The encoded output of standard unit is applied to non- inverting and inverting amplifiers 705, 706 selectively gated by generator 707 in synchronism with the horizontal blanking signals from 704 and in accordance with video polarity sense pulses derived from detectors 708 receiving the decoding and reset bursts from 701. Accounting signals are transmitted in the audio frequency spectrum, Fig. 10a (not shown). Details of the circuitry employed in the preferred embodiment are described with reference to those Figures unreferred to of the 14 Figs. accompanying the Specification.